1. Field of the Invention
The invention relates to amorphous metal tapes and somewhat more particularly to a method and device for manufacture of amorphous metal tapes.
2. Prior Art
Methods of manufacturing amorphous metal tapes directly from a suitable metal melt are known. Amorphous tapes are produced by quickly quenching a suitable metal melt at a velocity of about 10.sup.4 through 10.sup.6 .degree. K./s so that solidification without crystallization occurs. In these processes, molten amorphous metal alloys are typically extruded under pressure through one or more nozzle openings and the emerging molten metal stream is directed against a moving cooling surface. For example, the inner or outer surface of a rotating drum or of a travelling endless belt can be utilized as a cooling surface. The thickness of a tape obtained in this manner can, for example, amount to a few hundredths of a millimeter and the width can amount to a few millimeters and up to several centimeters.
Amorphous metals or alloys can be distinguished from crystalline metals or alloys by X-ray difraction measurements. In contrast to the crystalline materials, which exhibit characteristic sharp difraction lines, the intensity in X-ray difraction images of amorphous metal alloys changes only slowly with the difraction angle, somewhat similar to liquids or common glass.
Depending on the manufacturing conditions, tapes produced from amorphous alloys can be completely amorphous or can comprise a two-phase mixture of the amorphous and the crystalline states. In general, the phrase "amorphous metal alloy", as used in this art and in the instant specification and claims, defines an alloy whose molecular structure is at least 50 percent and preferably at least 80 percent amorphous.
It is already known to utilize round nozzle openings having a diameter of 0.5 through 1 mm in the manufacture of very narrow amorphous metal tapes. With this type of nozzle opening, a molten metal stream is expressed through such opening and strikes a moving surface of a cooling body after a free path of about 1 through 20 mm and expands thereon into a stationary molten drop. The desired metal tape grows from the underside of such drop due to advancing solidification. However, this process cannot be transferred without further ado, for example, for use with larger nozzle openings required to manufacture wider metal tapes because the tape geometry depends very greatly on the dimensions of the molten drop. With nozzle openings which are too large, the molten drop becomes too long and thus unstable at a correspondingly higher velocity of the cooling body surface. Further, the tape quality is adversely affected by all oscillations and the like in the free molten metal drop. The smooth and uniform surfaces required in broader tapes, as well as a uniform thickness and width over the entire length of a tape cannot be achieved with this technique.
German Offenlegungsschrift No. 27 46 238 suggests another method for producing amorphous metal tapes. In this process, a slotted nozzle connected to a supply container or crucible for molten metal is positioned in direct proximity, for example, at a distance of 0.03 through 1 mm, of a surface of a suitable cooling body. The width of the nozzle slot, as measured in the direction of motion of the cooling surface, is about 0.2 to a maximum of 1 mm. The width of the nozzle edges at both sides thereof are said to be particularly critical. The first edge, positioned in the direction of motion of the cooling surface, has a width which is at least equal to the width of the slot while the width of the second edge is about 1.5 through 3 times the width of the slot. Additionally, the distance between the nozzle opening and the cooling surface ranges between a 0.2 multiple to a 1 multiple of the slot width. With such parameters, the molten metal stream expressed from such nozzle opening forms a solidification front upon contact with the moving surface of the cooling body and such front passes directly past the second edge of the nozzle without contact. The flow velocity of the molten metal is primarily controlled by the viscous flux between the first edge of the nozzle and the solidified metal tape. However, nozzles with such small dimensions require extremely pure melts. Otherwise, there is a danger that the nozzle opening will be blocked due to incompletely dissolved or prematurely solidified particles of the melt. In addition to the relatively low production rates which are generally attained with narrow nozzle openings, a further disadvantage of this technique is that a significantly greater processing outlay is required in order to produce such narrow nozzle openings with the appropriate tolerances.